Power semiconductor devices, such as converters and inverters, require low loss, low power consumption, a high speed, and high efficiency. For example, a power diode or a power IGBT (insulated gate bipolar transistor) with a breakdown voltage of 600 V, or 1200 V or more is used as the power semiconductor device. Therefore, a power diode or a power IGBT which has low loss characteristics when it is turned on or switched has been developed.
Next, the impurity concentration distribution of a device, such as a diode or an IGBT, according to the related art will be described. FIG. 11 is a diagram illustrating the uniform impurity concentration distribution of a general diode according to the related art. FIG. 12 is a diagram illustrating the impurity concentration distribution of a diode with a broad buffer structure according to the related art. FIG. 11(a) schematically illustrates the cross-sectional structure of the general diode. FIG. 12(a) schematically illustrates the cross-sectional structure of the diode with the broad buffer structure (hereinafter, referred to as a BB structure) according to the related art.
As an improvement in the device which has low loss characteristics when it is turned on or switched, a device, such as a power diode or a diode or an IGBT with a BB structure in which a BB region 10 is provided in a drift layer 1 as illustrated in FIG. 12, has been proposed (for example, see the following Patent Document 1). The BB structure has an impurity concentration distribution in which impurity concentration is the maximum in the vicinity of the center of the n− type drift layer 1 in a PIN (P-Intrinsic-N) diode or IGBT and is slowly reduced toward a p anode layer 2 and an n+ cathode layer 3.
The drift layer 1 of the diode with the BB structure illustrated in FIG. 12 is compared with the drift layer (hereinafter, referred to as a drift layer with uniform impurity concentration) 1 with a uniform impurity concentration distribution in the general diode according to the related art illustrated in FIG. 11. The diode illustrated in FIG. 11(a) is configured so as to have the same thickness and breakdown voltage as those in the diode with the BB structure illustrated in FIG. 12. FIG. 11(b) illustrates the impurity concentration distribution taken along the cut line B-B′ of FIG. 11(a). FIG. 11(c) illustrates the impurity concentration distribution taken along the cut line A-A′ of FIG. 11(a).
In FIG. 12(b), the impurity concentration distribution taken along the cut line B-B′ of FIG. 12(a) is represented by a solid line. In FIG. 12(c), the impurity concentration distribution taken along the cut line A-A′ of FIG. 12(a) is represented by a solid line. In FIGS. 12(b) and 12(c), the impurity concentration distribution of the drift layer 1 with uniform impurity concentration in the general diode is represented by a dotted line in order to clarify the difference between the impurity concentration distribution of the drift layer 1 in the diode with the BB structure and the impurity concentration distribution of the drift layer 1 with uniform impurity concentration in the general diode.
As one of the methods of forming the BB region in the drift layer of the device, a method has been proposed which forms the BB region in the drift layer of the device using irradiation with protons (hereinafter, referred to as H+) (for example, see the following Patent Document 2). In the method of forming the BB region disclosed in Patent Document 2, an FZ (Floating Zone) bulk wafer is irradiated with H+ and a heat treatment is performed to partially change the radiated H+ into donors, thereby forming the BB region 10 with a desired impurity concentration distribution illustrated in FIGS. 12(b) and 12(c) in the drift layer 1.
However, the diode or IGBT with the BB structure has the following problems. As illustrated in FIGS. 11 and 12, in the general diode and the diode with the BB structure which have the same breakdown voltage, when the impurity concentration of the drift layer 1 with uniform impurity concentration is compared with the impurity concentration of the drift layer 1 having the BB region 10 provided therein, the maximum value (hereinafter, referred to as maximum impurity concentration) of the impurity concentration of the drift layer 1 having the BB region 10 provided therein is more than the impurity concentration of the drift layer 1 of the general diode. The minimum value of the impurity concentration of the drift layer 1 having the BB region 10 provided therein is less than the impurity concentration of the drift layer 1 of the general diode.
As such, in the drift layer 1 of the diode with the BB structure, the impurity concentration of a surface portion of the semiconductor substrate is less than that in the drift layer 1 with uniform impurity concentration in the general diode which has the same breakdown voltage as the diode with the BB structure. Therefore, it is easy for a depletion layer to extend in a direction (hereinafter, referred to as a surface direction) parallel to the surface of the semiconductor substrate in the surface portion of the semiconductor substrate and it is necessary to increase the area of a termination breakdown voltage region. When the impurity concentration of the surface of the termination breakdown voltage region is low, the device is likely to be affected by external charge. Therefore, the reliability of the breakdown voltage is likely to be reduced. As a method of avoiding the problem of the reliability of the breakdown voltage being reduced, a diode has been proposed in which a surface portion of a termination breakdown voltage region has high impurity concentration (for example, see the following Patent Document 3).